The present invention relates generally to a system and a method for providing fluid for immersion lithography and, more particularly, to offset partial ring seals for partial sealing of a gap between the optical element and the immersion nozzle that provides fluid delivery and recovery for immersion lithography.
An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly, a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultraviolet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left.
Immersion lithography is a technique which can enhance the resolution of projection lithography by permitting exposures with numerical aperture (NA) greater than one, which is the theoretical maximum for conventional “dry” systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer), immersion lithography permits exposure with light that would otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where only the final optical element and its housing and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography.
FIG. 1 shows an immersion lithography system 10 including a reticle stage 12 on which a reticle is supported, a projection lens 14, and a wafer 16 supported on a wafer stage 18. An immersion apparatus 20, which is sometimes referred to herein as a showerhead or a nozzle, is disposed around the final optical element 22 of the projection lens 14 to provide and recover a fluid, which may be a liquid such as water or a gas, between the final optical element 22 and the wafer 16. In specific embodiments, the immersion lithography system 10 includes the reticle and the wafer 16 that are moved synchronously in respective scanning directions during a scanning exposure. One of the challenges of immersion lithography is to design a system for delivery and recovery of a fluid, such as water, between the final optical element and the wafer, so as to provide a stable condition for immersion lithography.
FIG. 2 shows a partial cross-sectional view of a portion of the immersion lithography system 10 illustrating the relationship between the final optical element 22 and the immersion apparatus 20. To avoid transmitting vibration or other disturbances from the immersion apparatus 20 to the projection lens 14, a nozzle gap or spacing 24 is provided between the immersion apparatus 20 and the final optical element 22. During delivery and recovery of the fluid by the immersion apparatus 20 between the final optical element 22 and the wafer 16, and particularly during movement of the wafer stage 18 with respect to the projection lens 14 during scanning, the immersion fluid may escape through the gap 24.